Heretofore, various methods have been proposed for the control of a coagulant to be added in the dehydration of sludge. It has been proposed, for example, to control the amount of a coagulant to be added by indirectly measuring the residual polymer content of the liquid with the aid of a viscosimeter in view of the fact that the viscosity of the liquid varies with the residual polymer content and then using the result of this measurement as the criterion for the control.
The physical properties of the liquid include specific heat, electric conductivity, etc. besides the viscosity. They bring about various effects on the operation of addition of the coagulant. When the amount of addition of the coagulant is controlled by using as a sole criterion the viscosity which is only one of the physical properties, therefore, the actual amount of addition deviates from the proper level possibly to the extent of entailing ineffective coagulation, excessive addition of the coagulant, and unduly high cost of the coagulant.